FR2757507A1 - PROCESS FOR SEPARATING PARAXYLENE COMPRISING ADSORPTION WITH WATER INJECTION AND CRYSTALLIZATION - Google Patents

Abstract

The invention concerns a method for separating paraxylene on a simulated fluid bed from a C8 feedstock comprising a step of adsorption on a zeolite and of desorption by a desorption agent delivering an extract and a raffinate and a step of crystallisation of the extract. A water supply is introduced in the feedstock, in the desorption agent and/or in a flux recycling current in the column, such that the weighted mean of water contents measured in the extract and in the raffinate ranges between 1 and 200 ppm. The S/F ratio of the flow rate of the desorption agent over that of the feedstock during the step of adsorption and desorption ranges between 0.6 and 2.5.

Description

The invention relates to a process for the separation of paraxylene with improved producibility and reduced operating cost, from a mixture of aromatic C8 isomers containing it.

It applies to the preparation of paraxylene of very high purity for the synthesis of terephthalic acid, an intermediate in the nylons industry.

The combination of simulated countercurrent adsorption separation and crystallization has been described in US Pat. No. 5,401476 incorporated as a reference to provide more economical production of high purity paraxylene in a single step. The basic principles of this combination are as follows
- In the simulated moving bed separation stage, adjustments are used
different from the direct production of high purity paraxylene. The
flow rate is such that the flows in Zone 2 and Zone 3 (with flow rate
load = flow zone 2 - flow rate zone 3) no longer allow production
simultaneous extraction of an extract containing pure paraxylene and a raffinate
of paraxylene. We increase the load flow (therefore productivity) in
lowering the flow in zone 2. It therefore becomes impossible to obtain the
pure paraxylene (+ 98%). In addition, the solvent flow is also
decreased compared to a high purity setting especially in
increasing the flow in zone 4 beyond the shutdown flow. The increase of
productivity and lower operating costs related to the employment of less than
solvent are at the expense of purity.

In the crystallization stage, a mixture containing from 75% to 98% is treated.
% of paraxylene and more preferably 85 to 95% of paraxylene of
to reuse pre-existing crystallizers of units based on
crystallization schemes in two main stages. We operate from
preferably with a crystallization temperature of -15 ° C to + 15 ° C
in order to no longer consume frigories at low thermal level. In
the most favorable cases, an additional synergy is found in
using only one of those of the direct production of paraxylene
high purity alone and same solvent as desorbent in the stage
of adsorption and as a solvent for rinsing crystals in the
crystallization. However, when the adsorption step of the
charge on the adsorbent with a high charge desorbent ratio, by
example greater than 1.3, we achieve performance in purity of the product
sought very high, usually above 95%. The purity of the
product is high, the more it can fluctuate. So we can easily imagine that
since the crystallization purification step, which allows
to reach a purity higher than 99%, is calculated for a load of
purity determined and not subject to fluctuations, the
crystallizer is disturbed.

US Patent 3,734,974 discloses the use for simulated moving bed paraxylene (presumed to be of high purity in the absence of any mention of low purity paraxylene) of X or Y zeolites exchanged with Group IA cations. and / or group IIA (or a combination of both) with a controlled water content on the zeolite (from 1 to 5%). The desorbents used are for example toluene, or paradiethylbenzene or the diethylbenzenes in mixture or a mixture of these constituents with a paraffinic cut.

The advantage of the addition of water, in particular on K BaX and BaX zeolites, is a very sharp increase in the selectivities paraxylene-ethylbenzene and paraxylene-metaxylene.

US Pat. No. 4,778,946 describes, for the separation of ethylbenzene-metaxylene in feeds free of paraxylene, the use of zeolite KY containing up to 10% of water and up to 4% of either methanol or ammonia. so as not only to maximize the ethylbenzene-metaxylene selectivity but also to obtain a desorbent-ethylbenzene selectivity as close as possible to 1. The desorbent / filler ratio used is 2/1. This document specifies that a desorbent which is too strongly adsorbed does not allow good separation and that a desorbent which is too weakly adsorbed leads to an excessive demand for desorbent. It recommends that the ethylbenzenemetaxylene selectivity should be at least 3.0 and that the ethylbenzene desorbent selectivity be between 1 and 2.

US Pat. No. 5,401,476 does not suggest working in the presence of water, the hydrocarbons being hydrous.

US Patent 3,734,974 does not suggest the value of injecting water into a system where high purity paraxylene is not sought and does not recognize the role that might be water on paraxylene-desorbent selectivity.

No. 4,778,946 teaches that water can modify the ethylbenzene-desorbent selectivity in the case of only paraxylene-free feeds. Neither US Pat. No. 3,734,974 nor US Pat. No. 4,778,946 suggests a simple and practical means not to control the water content in the zeolite but rather to seek to control the water content in the hydrocarbons in contact with the zeolite.

The objects of the invention are to overcome the disadvantages of the prior art.

More specifically, the first object of the invention is to provide a high purity paraxylene separation process by combining a simulated moving bed adsorption step and a crystallization step, where the step of Simulated mobile bed adsorption has the particularity of working at a reduced charge solvent rate by means of continuous water injection carried out in the streams supplying the adsorption column or columns.

The second object of the invention is to favorably change the para-xylene-desorbent selectivity so as to obtain an optimum value making it possible to reduce the desorbent demand. What is surprising to one skilled in the art is that the injection of water does not produce an increase in purity with constant yield and productivity, nor an increase in yield at purity and fixed productivities, nor an increase in productivity with constant yield and purity, but a decrease in desorbent demand with constant purity and productivity.

A third object of the invention is to optimize the water content in the hydrocarbon effluents as a function of the nature of the adsorbent zeolite and the compensation cations and as a function of the temperature.

It has been observed that by introducing a suitable amount of water into the adsorption columns through the streams that enter therein, in combination with a quantity of desorbent limited in relation to the amount of charge, very good results.

More specifically, the invention relates to a process for separating paraxylene from a feedstock comprising a mixture of aromatic C8 isomers containing it, comprising a step of adsorbing and desorbing the isomers of the mixture on at least one column containing a zeolite, the adsorption and desorption step delivering under suitable conditions by means of a desorbent, a paraxylene-rich fraction and a paraxylene-poor fraction, the process further comprising at least one crystallization step of the fraction; rich in paraxylene which delivers pure paraxylene, the process being characterized in that a flow of water into the feedstock and / or into the desorbent and / or into a recycle stream of a flow in the column, such as that the weighted average of the water contents measured in the fraction rich in paraxylene and in the fraction poor in paraxylene is between 1 and 200 ppm and advantageously included in 3 and 120 ppm (parts per million) and in that the S / F ratio of the desorbent flow rate to that of the feedstock during the adsorption and desorption step is between 0.6 and 2.5, advantageously between 0.8 and 1.5 and preferably between 1 and 1.35.

Optimum results have been found resulting from the above combination depending on the nature of the zeolite, its compensation cations and the operating temperature.

Thus, according to a first embodiment of the process, it is possible to carry out the adsorption and desorption step at a temperature of 140 to 1600 ° C. on a zeolite Y exchanged with barium and potassium, the said average weighted water contents being between 3 and 6 ppm and the ratio S / F being between 1.15 and 1.35.

According to a second embodiment, the adsorption and desorption step can be carried out at a temperature of 165 to 185 ° C. on a zeolite Y exchanged with barium and potassium, said weighted average of the water contents being between 6 and 12 ppm and the S / F ratio being between 1.10 and 1.35.

According to a third embodiment, the adsorption and desorption step can be carried out at a temperature of 140 to 160 ° C., on a zeolite X exchanged with barium, said weighted average of the water contents being between 45.degree. and 60 ppm and the S / F ratio being between 1 and 1.25.

According to a fourth embodiment, the adsorption and desorption step can be carried out at a temperature of 165 to 185 ° C. on a barium-exchanged X zeolite, said weighted average of the water contents being between 60 and 130 ppm, preferably between 80 and 100 ppm and the S / F ratio being between 0.95 and 1.2.

According to a fifth embodiment, the adsorption and desorption step can be carried out at a temperature of 140 to 160 ° C. on a Y or X zeolite exchanged with potassium, said weighted average of the water contents being between 5 and 10 ppm and the S / F ratio being between 1.2 and 1.4.

According to a sixth embodiment, the adsorption and desorption step can be carried out at a temperature of 165 to 185 ° C. on an X zeolite exchanged with potassium, said weighted average of the water contents being between 10 and 20 ppm and the S / F ratio being between 1.2 and 1.4.

According to a seventh mode of implementation, the adsorption and desorption step can be carried out at a temperature of 110 to 1300 ° C. on a barium-exchanged X zeolite, said weighted average of the water contents being between 20 and 30 ppm. and the S / F ratio being from 1.2 to 1.4.

 It is well accepted that the paraxylene rich fraction and the paraxylene poor fraction can be distilled to remove the desorbent. The solvent-poor fraction freed from the desorbent optionally containing water in a small amount can be isomerized according to well-known isomerization processes and the paraxylene-enriched isomerate, optionally freed from light compounds, can be recycled, at least in part, to the adsorption and desorption zone.

According to one characteristic of the process, the fraction rich in paraxylene is crystallized, most often at high temperature, for example greater than -30 ° C. according to US Pat. Nos. 5,401,476 and WO 96,20,908 incorporated as references, describing crystallization steps. at one or more temperature levels.

The solvent generally used is toluene or paradiethylbenzene, for example. It can be recycled at least partly to the substantially anhydrous adsorption and desorption step.

According to another characteristic, it may be advantageous to introduce methanol in an amount generally less than 500 ppm, into the paraxylene-rich fraction freed from the desorbent but containing a minimal amount of water, which is intended to be crystallized. The crystallization stage then delivers a mother liquor containing water and methanol, which is removed for example by adsorption, and at least a portion of said mother liquor thus freed from water and methanol is recycled to the adsorption and desorption step.

The water contents in the hydrocarbon phases are naturally related to the water contents on the zeolite by an adsorption isotherm which does not depend substantially on the nature of the adsorbed hydrocarbon. As indicated in the prior art, it is necessary to distinguish between the water adsorbed on the zeolite that is measured by a loss on ignition at 400 ° C. and that the prior art measures by a loss on ignition at 500 ° C. ( referred to as Relative Volatile Free Basins) and between the much more strongly retained water that is measured by a loss on ignition at 900 ° C or 1000 ° C <Loss on ignition. When measured at 900 ° C or 1000 ° C, the structure of the zeolite is destroyed and it can be considered that the desorbed water difference between 400 ° C and 1000 ° C represents water of constitution . The difference between these two terms is of the order of 1.5 to 2% by weight on the faujasites.

The measurement of the water adsorption isotherm is carried out as follows: a batch of solid to be tested is allowed to be hydrated in the ambient atmosphere. Several columns of one meter in length and one centimeter in diameter (78.5 cm3) are filled with this zeolite and these columns are placed in an oven at 250 ° C. under a stream of very dry nitrogen (less than 10 ppm of water) Each of these columns is allowed to dehydrate for different times to obtain various water content (measured by weighing).

Each column is then placed in a closed loop comprising a dry hydrocarbon stock (of minimum volume), a positive displacement piston pump for liquid chromatography and the column. The hydrocarbon pool is equipped with a sampling port so as to measure the water content of the hydrocarbon once equilibrium is reached. The total amount of hydrocarbon is 100 cc. In this way, the amount of water contained in the hydrocarbon is negligible compared to that remaining retained on the zeolite.

Thus, the adsorption isotherms presented in FIGS. 1 and 2 relating respectively to zeolites KBaY and BaX measured by the loss on ignition at 400 ° C representing the amount of water (%) adsorbed by the zeolites as a function of the concentration (ppm) of water in the hydrocarbon phase at different temperatures.

The influence of the water content on adsorption separation is studied completely independently of the isothermal measurement. In this way, one avoids having to keep cumulative records of water inflows and outflows in the separation unit on which the accumulated errors are often very important.

The separation unit is filled with zeolite previously activated in an oven rotating at the time of the final phase of manufacture. According to the production batches, the loss on ignition at 400 ° C of the zeolite before loading is between 2% and 5% During the loading, the adsorbent partially rehydrates to an indeterminate degree (always less than 7% ).

When it is desired to work under hydrous conditions, dry desorbent is passed over the zeolite until only water contents of less than 1 ppm are measured on the effluents of the unit. This operation requires a significant amount of time (2 to 3 weeks).

On the other hand, when it is desired to work with known water content, one proceeds by injecting two controlled flow rates of desorbent, one of hydrous product, the other of water-saturated product and a controlled flow rate of anhydrous feed (one can also do the opposite). For example, if it is desired to control an average water content of 50 ppm on the incoming flows in the unit with flow rates of 5 cm3 / min respectively. load and 7 cm3 / min. desorbent, is injected 5 cm3 / min. anhydrous charge, 5.6 cc / min. of anhydrous solvent and 1.4 cm3 / min. of solvent saturated with water at ambient temperature (430 ppm in the case of toluene).

These conditions are maintained until the weighted average of the water contents at the outlets is substantially 50 ppm; for example, if respective water contents of 43 ppm are obtained on an extract rate of 5.3 cm 3 min. and 54 ppm on a raffinate flow of 6.65 cm3, the weighted average on the outputs is 49 ppm which is considered acceptable given the accuracy of the moisture content measurements.

To measure the water content of the incoming and outgoing flows, the KARL FISCHER method is used for contents greater than 15 ppm. When these levels are less than 15 ppm, the on-line measurements delivered by the on-line analysis probes (PANAMETRIC apparatus, series 1) are relied upon. A calibration is carried out between 15 ppm and 200 ppm, the extrapolation of this calibration curve for contents between 1 and 15 ppm is considered valid.

The following examples comparatively describe the use of the KBaY-toluene zeolite system with and without water, the KBaY zeolite system.
PDEB with or without water, BaX-toluene system with and without water and BaX-PDEB system with and without water. The appended figures are as follows - FIGS. 1 and 2 represent the quantity of water adsorbed by various
zeolites as a function of the concentration (ppm) of water in the phase
hydrocarbon at different temperatures; FIGS. 3, 4 and 5 show the solvent ratio with constant performances in
the weighted average of the amounts of water (ppm) contained in
outputs (extract and raffinate), at different temperatures; FIGS. 6, 7 and 8 show a performance index as a function of the
weighted average of the water contents in the outlets, expressed in ppm and - Figure 9 represents a relative performance index for various desorbents in
function of the desorbent charge ratio (S / F).

Example n01
A pilot unit for continuous liquid chromatography was carried out from 24 columns in series 1 m long and 1 cm in diameter, the circulation between the 24th and the first column being carried out by means of a recycling pump. At each intercolumn link, one can inject either a charge to be separated or solvent. You can also extract either a raffinate or an extract. This unit is described in a book titled Preparing and producing scale chromatography processes with applications, edited by G.

Barker, G. Ganestsos, chapter (From Batch Elution to Chronic Counter Current Chromatography by B. Balannec and G. Hotier, (Marcel's Publication
Dekker Inc., New York 1992). The adsorbent consists of zeolite Y exchanged with potassium and barium, the exchange rate expressed in normality is about 50% for each of the two cations. The zeolite is used in the form of beads 0.315 to 0.5 mm in diameter. All columns and winnowing dispensing is placed in an oven at 150 "C.

According to the principle of simulated countercurrent chromatography, three columns are advanced every six minutes at the co-current of the liquid circulation, the solvent injection, the extraction of the extract, the injection of the charge and raffinate removal.

According to the invention, the number of beds to be considered is therefore eight. Six columns (thus two beds) are between the solvent injection and the extraction of extract, nine columns (three beds) are between the extraction of the extract and the charge injection, three columns (one bed) are between the charge injection and the raffinate sample and the last six columns (two beds) lie between the raffinate sample and the solvent injection. The performance of the simulated moving bed unit is observed as a function of the weighted average of the water contents of the extract and the raffinate, the water having been introduced continuously into the columns via anhydrous desorbent and saturated desorbent as described above. For the first point of the curve of Figure 3, we recall the conditions of Example No. 01 of US 5,401,476. 7.2 cc / min of toluene and 5 cc / min of feed are continuously injected (expressed at ambient conditions). 5.40 cm 3 / min of extract and 6.74 cm 3 / min of raffinate are also continuously withdrawn; we see about 5% of losses. During the first period of the cycle which has eight, the solvent is injected in column 1, the extract is taken at the outlet of column 6, the feed is injected in column 15, the raffinate is taken at the outlet of the column 18. During the first two periods of the cycle, the recycling pump delivers at room temperature 38.7 cm3 / min; during the third period, it delivers at 45.5 cm3 / min; during the next three periods it delivers at 40.5 cm3 / min and during the last two periods it delivers 45.9 cm3 / min. So we have an average recycling rate of 42 cm3 / min. Paraxylene is obtained with a purity of 92.2% and a recovery rate of 98.1%. The pressure decreases approximately linearly from 30 bar to 5 bar. The following table gives the stationary steady state of the unit

<tb><SEP> Load <SEP> Solvent <SEP> Extract <SEP> Raffinate
<tb> Flow rate <SEP> 5 <SEP> cm3 / min <SEP> 7.2 <SEP> cm3 / min <SEP> 5.40 <SEP> cm3 / min <SEP> 6.74 <SEP> cm3 / min
<tb> Toluene <SEP> 99.9 <SEP>% <SEP> 79.30 <SEP>% <SEP> 43.29 <SEP>%
<tb> Ethylbenzene <SEP> 17 <SEP>% <SEP> 1.07% <SEP> 11.72% <SEP>
<tb> M <SEP> Xylene <SEP> 4 <SEP>% <SEP> 0.40 <SEP>% <SEP> 32.30 <SEP>%
<tb> Xylene <SEP> 18% <SEP> 015% <SEP> 1240% <SEP>
<tb> P <SEP> Xylene <SEP> 21 <SEP>% <SEP> 19.08% <SEP> 0.29 <SEP>%
<Tb>
A first way to operate the unit is to reduce the desorbent flow (toluene) while keeping the flow rate constant. We try to keep purity and efficiency almost constant, this is achieved by playing on the extract-raffinate balance, the flows in zones 2 and 3 as well as the time of the permutations are kept constant, the flows in the zones 1 and 4 are adjusted by increasing the flow rate in zone 4 and reducing the flow rate in zone 1.

This way of operating is close to the type of operation on an industrial unit, however it is difficult to get points at isopuracy and isorendement. Purity and yield variations of the order of 1% are tolerated provided that the geometric mean of purity and yield does not vary more than 0.3%.

FIG. 3 shows that the solvent flow rate is minimized at 150 ° C for an average water content of 5 ppm: the solvent / filler ratio is 1.27 / 1 while for an anhydrous solvent (US 5,401,476) it stands at 1.44 / 1.

Example 2 (Fig. 4)
The experiment is repeated by increasing the operating temperature from 150 ° C. to 175 ° C. The purity and yield performances are kept identical with respect to the 150 ° C. experiments for almost identical solvent levels for the anhydrous zeolite and for the optimum. of water content. However, Figure 4 shows that the optimum water content is not 5 ppm but 10 ppm.

This corresponds to an optimum water content on the zeolite of 0.4 to 0.5% (loss on ignition at 400 ° C), which is impossible to measure with a correct accuracy directly.

Example 3 (Fig. 5)
The experiment at 175 ° C is repeated with 98% purity paradiethylbenzene desorbent (the impurities consist mainly of meta and orthodiethylbenzene) Figure 5 shows that the solvent content can be reduced by approximately 0.1 / 1 and that the optimum performance is still 10 ppm water in the hydrocarbon.This gain in solvent content is related to the PX-desorbent selectivity which is 0.55 in the case of toluene and 0.72 in the case of paradiethylbenzene.

Example 4 (Fig. 6)
The same unit is loaded with BaX zeolite in the form of beads 0.3 to 0.8 mm in diameter containing 22% clay-based binder. The residual sodium content after exchange is less than 3% of the cations (expressed in normality).

Only 12 columns are used instead of 24 and the permutations are all columns instead of all 3 columns as before. There are 3 beds in each zone. The permutation time is one minute, the ratio between the average recycling flow rate and the feedstock is 4.5 / 1, the solvent content is kept constant at 1.25 / 1, the ratio extracted on raffinate is 0, 40/1 and the charge rate is 8 cm3 / min. Productivity is much higher compared to the previous case thanks to a reduction of the pressure drop. The unit is operated at 175 ° C and the desorbent is paradiethylbenzene.

We define a performance index

<tb> IP <SEP> = <SEP> ss / yield <SEP>% <SEP> x <SEP> Purity <SEP>% <SEP>.
<Tb>

The flows in the zones are slightly adjusted so as to keep a reasonable balance between purity and yield (never more than 6% difference). FIG. 6 shows that the performance index on the BaX zeolite is maximum for a water content approximately ten times higher than with the KBaY zeolite. On the other hand, the performance of the BaX zeolite is significantly lower than that of the KBaY zeolite when these products are anhydrous while they are significantly higher, the optimum water content, which is not taught by the prior art.

Example (Fig. 7)
The same experiment is repeated at 150 ° C. In addition to a considerably higher pressure drop (increase of about 70%), there is lower performance and a different optimum of water content in the hydrocarbon (50 to 60 ppm). (see Fig. 7)).

Example 6 (Fig. 8)
Examples 4 and 5 are repeated except that the solvent is toluene and the temperature of 120 ° C: the maximum performance is obtained this time for a water content in the hydrocarbon (weighted average of the outlets) of about 25 ppm, the ratio extracted on raffinate is about 0.7 / 1 (see Fig. 8).

Example 7 (Fig. 9)
Example 4 is repeated, this time fixing the water content at the optimum, ie 90 ppm, and the solvent / filler ratio is varied. The feed contains only 4% ethylbenzene for 22.5% paraxylene. In addition, 23.5% of orthoxylene, 1.5% of toluene and 48.5% of metaxylene are present. This load being easier to treat, it can inject a higher flow rate is 10 cm3 / min. The ratio recycled on load is lowered to 4.4 / 1 and the permutation time is lowered to 50 seconds. It is observed that above a solvent / charge ratio of 1.45 / 1, the performances remain identical. By lowering the desorbent flow rate to 8 cm3 / min., It is realized that it is possible for solvent levels of less than 1 to obtain purities greater than 75% sufficient for crystallization purification which constitutes the second stage of the process (see Fig. 9).

When the PDEB desorbent is replaced by toluene, it is realized that the performances are slightly worse and that the flow rate settings are different (the flow rates in zones 1 and 4 are higher and the extract ratio on raffinate must be increased. The advantage of using toluene is to be able to treat feeds containing high levels of aromatic C9.

Depending on whether toluene or paradiethylbenzene is used, the water contained in the extract should be removed before sending the low purity paraxylene to the crystallization step.

When the desorbent used is toluene, the impure paraxylene is withdrawn (substantially free of water) at the bottom of the extract column. At the top of the distillation columns of extract and raffinate, liquid water is withdrawn through a settling tray and is carried out continuously or periodically lighter component purges than toluene. About 3 to 10 trays lower, the substantially anhydrous toluene is drawn off on a racking tray.

When the desorbent is paradiethylbenzene and the charge to be treated contains a little toluene, it is also possible to operate in this way (in this case at the top of the distillation column, water is withdrawn and

Claims (13)

A process for separating paraxylene from a feedstock comprising a mixture of aromatic C8 isomers containing it, comprising a step of adsorbing and desorbing the isomers of the mixture on at least one column containing a zeolite, the step adsorption and desorption process under suitable conditions using a desorbent, a paraxylene-rich fraction and a paraxylene-poor fraction, the process further comprising at least one step of crystallizing the paraxylene rich fraction which delivers pure paraxylene, the process being characterized by introducing a flow of water into the feedstock, into the desorbent and / or into a recycle stream of a flow in the column, such as the weighted average of the water contents measured in the fraction rich in paraxylene and in the fraction poor in paraxylene is between 1 and 200 ppm and advantageously between 3 and 120 ppm (parts per million) and in c e that the S / F ratio of the desorbent flow rate to that of the feedstock during the adsorption and desorption step is between 0.6 and 2.5, advantageously between 0.8 and 1.5, and preferred between 1 and 1.35. 2. The process as claimed in claim 1, wherein the adsorption and desorption step is carried out at a temperature of 140 ° C. to 1600 ° C. on a zeolite Y exchanged with barium and potassium, said weighted average of the water contents being between 3 and 6 ppm and the S / F ratio being between 1.15 and 1.35. 3. The process according to claim 1, wherein the adsorption and desorption step is carried out at a temperature of 165 to 185 ° C. on a zeolite Y exchanged with barium and potassium, said weighted average of the water contents being included. between 6 and 12 ppm and the S / F ratio being between 1.10 and 1.35. 4. Process according to claim 1, wherein the adsorption and desorption step is carried out at a temperature of 140 to 160 ° C, on a zeolite X exchanged with barium, said weighted average of the water contents being between 45 ° C. and 60 ppm and the S / F ratio being between 1 and 1.25. 5. The process as claimed in claim 1, in which the adsorption and desorption step is carried out at a temperature of 165 to 185 ° C. on a barium-exchanged X zeolite, said weighted average of the water contents being between 60 and 130 ppm, preferably between 80 and 100 ppm and the S / F ratio being between 0.95 and 1.2. 6. The process according to claim 1, wherein the adsorption and desorption step is carried out at a temperature of 140 to 160 ° C. on a Y or X zeolite exchanged with potassium, said weighted average of the water contents being between and 10 ppm and the S / F ratio being between 1.2 and 1.4. 7- The method of claim 1 wherein the adsorption and desorption step is carried out at a temperature of 165 to 185 ° C on a zeolite X exchanged with potassium, said weighted average of the water contents being between 10 and 20 ppm and the S / F ratio being between 1.2 and 1.4. 8- The method of claim 1, wherein the adsorption and desorption step is carried out at a temperature of 110 to 1300C on a zeolite X exchanged with barium, said weighted average water content is between 20 and 30 ppm and the S / F ratio being from 1.2 to 1.4. 9- Method according to one of claims 1 to 8 wherein the paraxylene-rich fraction and the paraxylene-poor fraction that result from the adsorption and desorption step are distilled to remove the desorbent, the paraxylene-rich fraction substantially free of desorbent being subjected to the crystallization step. 10- The method of claim 9 wherein the desorbent is toluene or paradiétylbenzène and is recycled at least in part substantially anhydrous in the adsorption and desorption step. 11- Method according to one of claims 9 to 10 wherein methanol is introduced into the paraxylene-rich fraction freed desorbent but containing a minimal amount of water, before the crystallization step. 12- The method of claim 11, wherein the crystallization step delivers a mother liquor containing water and methanol and wherein the water and methanol is removed from the mother liquor, before recycling at least into part of the adsorption and desorption step. 13- Method according to one of claims 9 to 12, wherein the paraxylene-poor fraction freed desorbent is isomerized.